The present invention relates to an antenna associated with a receiver or transmitter, for working with linearly polarized waves and comprising a device for transposing the direction of the linear polarization of the waves received by the receiver or transmitted by the transmitter.
The directional adjustment of the linear polarization received from an antenna by a receiver associated with the antenna is required in some cases. For example, in the case of land-based stations, the same vertically polarized wave transmitted by a satellite is picked up with a direction of its polarity which forms with the vertical an angle which depends, more especially, on the latitude and longitude of the place where this wave is picked up. Similarly, an adjustment is required when an antenna changes satellite for the waves which it receives. Because of the very principle of reciprocity, this is equally true for transmission from, for example, land-based stations.
Transposition devices are known which are formed by equipping the ground reception antenna with a mechanical or electro-mechanical system for adjusting the direction of polarization by causing all or part of the antenna to rotate. These rotations are quite compatible with Cassegrain optical antennae, where it is easy to provide, at the position where the horn is located, i.e. at the top of the main dish reflector, an attachment for causing the assembly formed by the horn and its connections to rotate. On the other hand, with prime focus antennae, the rotation of the primary source (horn+connections) for adjusting the polarization is practically impossible. In fact, in this case, the primary source is generally rigidly fixed at both ends: at the level of the radiating horn and at the level of the junction between its connections and the receiver(s) associated with the antenna. FIGS. 1 and 2 relating to the following description will show the difficulty in providing, in a prime focus antenna, a mechanical assembly for rotating the horn and its connections about the main axis of the reflector.
The object of the present invention is to provide an antenna equipped with a device for transposing the direction of polarization, both simple and inexpensive, able to be used not only for prime focus antennae but also for Cassegrain optical antennae.